The purpose of this study is to evaluate the growth and body comosition of preterm infants fed different formulas and/or human milk. The formulas vary in the type of fat they contain. Two important fatty acids present in human milk, but not commercially available in infant formulas are docosahexaenoic acid (DHA) and arachidonic acid (AA). This study will evaluate the effect of adding sources of DHA and AA to formulas. The GCRC Core Lab will be used to run the leptins.